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Characterization and stability of catanionic vesicles formed by pseudo-tetraalkyl surfactant mixtures† Carlotta Pucci,a Lourdes Pe´rez,b Camillo La Mesa*a and Ramon Pons*b The phase behavior of an ad hoc synthesized surfactant, sodium 8-hexadecylsulfate (8-SHS), and its mixtures with didecyldimethylammonium bromide (DiDAB) in water is reported. We dealt with dilute concentration regimes, at a total surfactant content of 98% RoseChemicals Ltd.). Scattering curves were smeared by the detector width. We used a detector-focused small beam (300 400 mm full width at half maximum), which widens the peaks without noticeable effect on its position. The background was subtracted from the scattering curves. The curves were scaled in absolute units by comparison with water scattering.20,21 The instrumentally smeared experimental SAXS curves were tted to numerical models, convoluted with beam size and detector width effects.22 A least squares routine based on the Levenberg– Marquardt scheme was used.23 The bilayer thickness was determined using a three-Gaussian prole based on the MCG model according to Pabst et al.24,25
Electrophoretic mobility measurements, m, were run at 25.0 0.1 C using a Laser-Doppler facility available in the DLS equipment. The dispersions were placed into U-shaped cuvettes, equipped with gold electrodes. The z-potential, z, is related to m by the relationship28 4ph (4) z¼m 30
Dynamic light scattering (DLS) Measurements were run using a Malvern Zetasizer unit, Nano ZS series HT, working at l ¼ 638.2 nm in back-scattering mode (at 173 ), at 25.0 0.1 C. A digital correlator analyzes the scattered light intensity uctuations, I(q, t), due to the Brownian motion of the dispersed particles, at times t and (t + s), where s is the delay time. The intensity autocorrelation function G2(q, t) was obtained according to # " hIðq; tÞ$Iðq; t þ sÞi G2 ðq; tÞ ¼ ¼ 1 þ Bjg1 ðtÞj2 (2) hIðqÞi2 where q is the scattering vector. G2(q, t) is related to the electromagnetic eld autocorrelation function g1(q, t) through the equation G2(q, t) ¼ A + B|g1(q, t)|2
(3)
where A is the baseline and B is the intercept. The function g1(q, t) is expanded by a cumulant analysis,26 where the rst term provides the diffusion coefficient, D, related to the hydrodynamic radius RH of the particles through the Stokes– Einstein equation. The second cumulant is proportional to the polydispersity index, PdI. Intensity distributions were obtained by analyzing the autocorrelation functions by CONTIN algorithms.27
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where h is the solvent viscosity and 30 is its static dielectric permittivity. In vesicular samples Smoluchowski's approximation holds, because the electrical double layer thickness surrounding vesicles is much smaller than their radius.
Results and discussion DiDAB characterization The critical association values determined by conductivity and surface tension measurements are similar to literature ones29,30 (see Table 1 and Fig. S1 in the ESI†). From the Gibbs isotherm ˚ 2. (eqn (1)), we obtained an area per surfactant molecule of 74 A We calculated the chain length, l, and the hydrophobic volume, V, by Tanford's formulae31 V ¼ m(27.4 + 26.9nCH2)
(5)
l ¼ 1.5 + 1.265nCH2
(6)
˚ and m is the number of hydrophobic ˚ 2 (or A) where sizes are in A chains. We deduced the packing parameter, P, from the relationship32 V P¼ (7) lAm and obtained a value of 0.6. Am is the area per polar head group. According to the theory, P values are compatible with the existence of vesicles.3 The observed phase sequence is: molecular solution, aggregates, and, at high surfactant concentrations, lamellar and hexagonal phases.32,33 The value of P increases accordingly. The phase sequence was conrmed by SAXS and polarized light microscopy (see ESI, Fig. S2 and S3 and Table. S1†). 8-SHS characterization The critical concentration of the above surfactant was determined by ionic conductivity, surface tension and potentiometric measurements, using a surfactant-selective electrode (see ESI, Fig. S4†). Data are summarized in Table 1. From surface tension measurements, the area per polar head ˚ 2. The corresponding packing of 8-SHS was estimated to be 99 A parameter, determined by eqn (5)–(7), is 0.4; therefore, rods or disks are the preferred geometries for 8-SHS micellar aggregates. A crucial point to be put in evidence from data in Table 1 is that the critical thresholds for 8-SHS obtained by different methods moderately agree with each other, but lie clearly out of the reproducibility limits of the different techniques. The
Soft Matter
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Soft Matter Table 1
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CMCs, area per polar head group, Am, and packing parameter P of DiDAB and 8-SHS, at 25.0 C
Published on 09 October 2014. Downloaded by University of Illinois at Chicago on 03/11/2014 20:19:09.
CMC (mmol kg1) Surfactant
Conductivity
Surface tension
Selective electrode
˚ 2 mol1) Am (A
P
DiDAB 8-SHS
1.4 0.1 1.3 0.1
1.4 0.15 2.0 0.2
— 2.8 0.3
74 5 99 5
0.6 0.4
differences observed among the reported methods could be ascribed to a not well-dened aggregation process, starting with the formation of dimers, and ending into micelles, or other entities. It is worth noticing, on this regard, that the different techniques we used detect aggregation by measuring not strictly related parameters; this could be another reason for the occurrence of slightly different critical values. For instance, potentiometric measurements carried out using a Na+ selective electrode (see ESI, Fig. S4D†) suggest that the aggregation process starts at a surfactant concentration close to 0.3 mmol kg1 and ends at z1 mmol kg1. Above this point, counterion binding to micelles, b, is constant and close to 0.55. Therefore, aggregate formation and growth depend on the molecular features of the surfactant. The hypothesis of a stepwise association for 8-SHS is realistic and has been shown to occur in surfactant with short hydrophobic chains.34 At 6% wt/wt in 8-SHS, giant vesicles are formed (see ESI, Fig. S5†). At higher concentrations (15, 20, and 25% wt), SAXS spectra show a broad maximum (see ESI S6†) with features peculiar to lipid bilayer structures.35,36 It is possible to identify the repetition distances from the position at 1 : 2 spacings; the results indicate the occurrence of the lamellar order at very small q values. We evaluated the Bragg repetition distances to ˚ for 15, 20 and 25% wt/wt, respectively. be 140, 115 and 85 A Fitting the experimental curves by a Caill´ e-modied anal˚ bilayer thickness in all cases considered, ysis24,36–38 gives a 23 A ˚ 2. Since the 8-SHS while the area per molecule is 51 2 A ˚ long, no hydrophobic chain in extended conformation is 11.6 A signicant chain interpenetration between the two leaets occurs. Therefore, ideal swelling occurs in the lamellar phase.
To determine the phase behavior of 8-SHS at high concentrations, phase scanning experiments by polarized optical microscopy were performed (Fig. 1). A drop of water was placed at the outer edge of the cover-slip and allowed to diffuse into the solid. The above procedure induces a concentration gradient and indicates a qualitative phase sequence.39,40 At a relatively high water content, a lamellar phase is met rst (Fig. 1A). Then an isotropic phase, presumably a cubic one (Fig. 1B), is observed. A univocal assignment on the isotropic phase structure is cumbersome. At still higher concentrations in 8-SHS it is possible to recognize fan-like textures. These are typical of a hexagonal phase and occur at concentrations close to the hydrated crystal threshold (Fig. 1C). The observed phase is tentatively assumed to be reversed hexagonal. Catanionic mixtures The formation of the catanionic systems is strongly synergistic. A clear effect is the decrease of critical thresholds compared to the individual surfactants (Table 2 and ESI, Fig. S7†). The geometry of the aggregates is not easily determined a priori, and we prefer dening it as the critical aggregation concentration, CAC, rather than critical micellization, or vesiculation. The above quantity depends on molar ratios. For DiDAB-rich mixtures, the decrease in CAC values is signicant, probably because this surfactant has a marked propensity to form vesicles. From CAC values by surface tension, we estimated the interaction parameter among the two surfactants, b, at different compositions, according to41–43 a1 CACmixt ln X1 CAC1 b¼ (8) ð1 X1 Þ2 where a1 is the mole fraction of the 1st surfactant, CACmix is the critical aggregation concentration of the given mixture, CAC1 is CAC, b, and DGagg for different mole fractions in DiDAB (XDiDAB), at 25.0 C
Table 2
Fig. 1 Polarized light micrographs of hydrated 8-SHS in phase scanning experiments, at 25.0 C. (A) Lamellar phase; (B) isotropic phase; (C) fan-like textures peculiar to a hexagonal phase. The water concentration decreases from (A) to (C), according to the direction indicated by the white arrow.
Soft Matter
XDiDAB (a1)
CAC (mmol kg1)
b (kBT)
DGagg (kJ mol1)
0 0.1 0.2 0.3 0.8 0.9 1
2.0 0.17 0.02 0.017 0.02 0.03 1.4
— 11 18.5 18.7 18.2 17.8 —
15 21.5 26.8 27.2 26.8 25.8 15.9
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that of component 1, and X1 is the mole fraction of component 1 in the aggregate. The latter can be determined by41 a1 CACmixt ðX1 Þ2 ln X1 CAC1 1¼0 (9) ð1 a1 ÞCACmixt 2 ð1 X1 Þ ln ð1 X1 ÞCAC2 where the meaning of each symbol is the same as above. The Gibbs energy of aggregation, DGagg, was calculated according to:44 DGagg ¼ RT ln CAC
(10)
The results obtained by combining eqn (8) and (10) are reported in Table 2. The interactions among the components in surfactant mixtures imply negative or positive deviations from ideality of mixing. The interaction parameter, b, indicates whether positive (anti-cooperative), or negative (cooperative) contributions to the Gibbs energy take place. For all mixtures considered here b is
Published on 09 October 2014. Downloaded by University of Illinois at Chicago on 03/11/2014 20:19:09.
PAPER
Cite this: DOI: 10.1039/c4sm01575d
View Journal
Characterization and stability of catanionic vesicles formed by pseudo-tetraalkyl surfactant mixtures† Carlotta Pucci,a Lourdes Pe´rez,b Camillo La Mesa*a and Ramon Pons*b The phase behavior of an ad hoc synthesized surfactant, sodium 8-hexadecylsulfate (8-SHS), and its mixtures with didecyldimethylammonium bromide (DiDAB) in water is reported. We dealt with dilute concentration regimes, at a total surfactant content of 98% RoseChemicals Ltd.). Scattering curves were smeared by the detector width. We used a detector-focused small beam (300 400 mm full width at half maximum), which widens the peaks without noticeable effect on its position. The background was subtracted from the scattering curves. The curves were scaled in absolute units by comparison with water scattering.20,21 The instrumentally smeared experimental SAXS curves were tted to numerical models, convoluted with beam size and detector width effects.22 A least squares routine based on the Levenberg– Marquardt scheme was used.23 The bilayer thickness was determined using a three-Gaussian prole based on the MCG model according to Pabst et al.24,25
Electrophoretic mobility measurements, m, were run at 25.0 0.1 C using a Laser-Doppler facility available in the DLS equipment. The dispersions were placed into U-shaped cuvettes, equipped with gold electrodes. The z-potential, z, is related to m by the relationship28 4ph (4) z¼m 30
Dynamic light scattering (DLS) Measurements were run using a Malvern Zetasizer unit, Nano ZS series HT, working at l ¼ 638.2 nm in back-scattering mode (at 173 ), at 25.0 0.1 C. A digital correlator analyzes the scattered light intensity uctuations, I(q, t), due to the Brownian motion of the dispersed particles, at times t and (t + s), where s is the delay time. The intensity autocorrelation function G2(q, t) was obtained according to # " hIðq; tÞ$Iðq; t þ sÞi G2 ðq; tÞ ¼ ¼ 1 þ Bjg1 ðtÞj2 (2) hIðqÞi2 where q is the scattering vector. G2(q, t) is related to the electromagnetic eld autocorrelation function g1(q, t) through the equation G2(q, t) ¼ A + B|g1(q, t)|2
(3)
where A is the baseline and B is the intercept. The function g1(q, t) is expanded by a cumulant analysis,26 where the rst term provides the diffusion coefficient, D, related to the hydrodynamic radius RH of the particles through the Stokes– Einstein equation. The second cumulant is proportional to the polydispersity index, PdI. Intensity distributions were obtained by analyzing the autocorrelation functions by CONTIN algorithms.27
This journal is © The Royal Society of Chemistry 2014
where h is the solvent viscosity and 30 is its static dielectric permittivity. In vesicular samples Smoluchowski's approximation holds, because the electrical double layer thickness surrounding vesicles is much smaller than their radius.
Results and discussion DiDAB characterization The critical association values determined by conductivity and surface tension measurements are similar to literature ones29,30 (see Table 1 and Fig. S1 in the ESI†). From the Gibbs isotherm ˚ 2. (eqn (1)), we obtained an area per surfactant molecule of 74 A We calculated the chain length, l, and the hydrophobic volume, V, by Tanford's formulae31 V ¼ m(27.4 + 26.9nCH2)
(5)
l ¼ 1.5 + 1.265nCH2
(6)
˚ and m is the number of hydrophobic ˚ 2 (or A) where sizes are in A chains. We deduced the packing parameter, P, from the relationship32 V P¼ (7) lAm and obtained a value of 0.6. Am is the area per polar head group. According to the theory, P values are compatible with the existence of vesicles.3 The observed phase sequence is: molecular solution, aggregates, and, at high surfactant concentrations, lamellar and hexagonal phases.32,33 The value of P increases accordingly. The phase sequence was conrmed by SAXS and polarized light microscopy (see ESI, Fig. S2 and S3 and Table. S1†). 8-SHS characterization The critical concentration of the above surfactant was determined by ionic conductivity, surface tension and potentiometric measurements, using a surfactant-selective electrode (see ESI, Fig. S4†). Data are summarized in Table 1. From surface tension measurements, the area per polar head ˚ 2. The corresponding packing of 8-SHS was estimated to be 99 A parameter, determined by eqn (5)–(7), is 0.4; therefore, rods or disks are the preferred geometries for 8-SHS micellar aggregates. A crucial point to be put in evidence from data in Table 1 is that the critical thresholds for 8-SHS obtained by different methods moderately agree with each other, but lie clearly out of the reproducibility limits of the different techniques. The
Soft Matter
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Soft Matter Table 1
Paper
CMCs, area per polar head group, Am, and packing parameter P of DiDAB and 8-SHS, at 25.0 C
Published on 09 October 2014. Downloaded by University of Illinois at Chicago on 03/11/2014 20:19:09.
CMC (mmol kg1) Surfactant
Conductivity
Surface tension
Selective electrode
˚ 2 mol1) Am (A
P
DiDAB 8-SHS
1.4 0.1 1.3 0.1
1.4 0.15 2.0 0.2
— 2.8 0.3
74 5 99 5
0.6 0.4
differences observed among the reported methods could be ascribed to a not well-dened aggregation process, starting with the formation of dimers, and ending into micelles, or other entities. It is worth noticing, on this regard, that the different techniques we used detect aggregation by measuring not strictly related parameters; this could be another reason for the occurrence of slightly different critical values. For instance, potentiometric measurements carried out using a Na+ selective electrode (see ESI, Fig. S4D†) suggest that the aggregation process starts at a surfactant concentration close to 0.3 mmol kg1 and ends at z1 mmol kg1. Above this point, counterion binding to micelles, b, is constant and close to 0.55. Therefore, aggregate formation and growth depend on the molecular features of the surfactant. The hypothesis of a stepwise association for 8-SHS is realistic and has been shown to occur in surfactant with short hydrophobic chains.34 At 6% wt/wt in 8-SHS, giant vesicles are formed (see ESI, Fig. S5†). At higher concentrations (15, 20, and 25% wt), SAXS spectra show a broad maximum (see ESI S6†) with features peculiar to lipid bilayer structures.35,36 It is possible to identify the repetition distances from the position at 1 : 2 spacings; the results indicate the occurrence of the lamellar order at very small q values. We evaluated the Bragg repetition distances to ˚ for 15, 20 and 25% wt/wt, respectively. be 140, 115 and 85 A Fitting the experimental curves by a Caill´ e-modied anal˚ bilayer thickness in all cases considered, ysis24,36–38 gives a 23 A ˚ 2. Since the 8-SHS while the area per molecule is 51 2 A ˚ long, no hydrophobic chain in extended conformation is 11.6 A signicant chain interpenetration between the two leaets occurs. Therefore, ideal swelling occurs in the lamellar phase.
To determine the phase behavior of 8-SHS at high concentrations, phase scanning experiments by polarized optical microscopy were performed (Fig. 1). A drop of water was placed at the outer edge of the cover-slip and allowed to diffuse into the solid. The above procedure induces a concentration gradient and indicates a qualitative phase sequence.39,40 At a relatively high water content, a lamellar phase is met rst (Fig. 1A). Then an isotropic phase, presumably a cubic one (Fig. 1B), is observed. A univocal assignment on the isotropic phase structure is cumbersome. At still higher concentrations in 8-SHS it is possible to recognize fan-like textures. These are typical of a hexagonal phase and occur at concentrations close to the hydrated crystal threshold (Fig. 1C). The observed phase is tentatively assumed to be reversed hexagonal. Catanionic mixtures The formation of the catanionic systems is strongly synergistic. A clear effect is the decrease of critical thresholds compared to the individual surfactants (Table 2 and ESI, Fig. S7†). The geometry of the aggregates is not easily determined a priori, and we prefer dening it as the critical aggregation concentration, CAC, rather than critical micellization, or vesiculation. The above quantity depends on molar ratios. For DiDAB-rich mixtures, the decrease in CAC values is signicant, probably because this surfactant has a marked propensity to form vesicles. From CAC values by surface tension, we estimated the interaction parameter among the two surfactants, b, at different compositions, according to41–43 a1 CACmixt ln X1 CAC1 b¼ (8) ð1 X1 Þ2 where a1 is the mole fraction of the 1st surfactant, CACmix is the critical aggregation concentration of the given mixture, CAC1 is CAC, b, and DGagg for different mole fractions in DiDAB (XDiDAB), at 25.0 C
Table 2
Fig. 1 Polarized light micrographs of hydrated 8-SHS in phase scanning experiments, at 25.0 C. (A) Lamellar phase; (B) isotropic phase; (C) fan-like textures peculiar to a hexagonal phase. The water concentration decreases from (A) to (C), according to the direction indicated by the white arrow.
Soft Matter
XDiDAB (a1)
CAC (mmol kg1)
b (kBT)
DGagg (kJ mol1)
0 0.1 0.2 0.3 0.8 0.9 1
2.0 0.17 0.02 0.017 0.02 0.03 1.4
— 11 18.5 18.7 18.2 17.8 —
15 21.5 26.8 27.2 26.8 25.8 15.9
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that of component 1, and X1 is the mole fraction of component 1 in the aggregate. The latter can be determined by41 a1 CACmixt ðX1 Þ2 ln X1 CAC1 1¼0 (9) ð1 a1 ÞCACmixt 2 ð1 X1 Þ ln ð1 X1 ÞCAC2 where the meaning of each symbol is the same as above. The Gibbs energy of aggregation, DGagg, was calculated according to:44 DGagg ¼ RT ln CAC
(10)
The results obtained by combining eqn (8) and (10) are reported in Table 2. The interactions among the components in surfactant mixtures imply negative or positive deviations from ideality of mixing. The interaction parameter, b, indicates whether positive (anti-cooperative), or negative (cooperative) contributions to the Gibbs energy take place. For all mixtures considered here b is
